The class II molecules of the MHC are highly polymorphic, and this polymorphism results in three recognition phenotypes: 1) Regulatory T cells recognize foreign antigens in the context of self class II and thus class II molecules are said to restrict T cell activation by antigen; 2) Polymorphic class II molecules themselves can elicit a humoral immune response; and 3) They can activate non-self T cells in the apparent absence of foreign antigen. Sequence data has revealed that the polymorphic regions of the class II HLA genes contain multiple substitutions in short stretches interspersed among long stretches of invariant sequence. The research proposed here is based on the generally accepted but not fully established hypothesis that the polymorphic sequences of class II molecules define the structure of a recognition epitode or epitopes. This hypothesis will be tested in HLA-DR beta chains by two approaches: 1. The first is a direct molecular genetic approach. By using oligonucleotide directed mutagensis, a polymorphic region from one DR beta chain gene will be transferred onto another. After transfection and expression of these genes in fibroblasts, the resulting hybrid DR chain will be studied to see if it gains a new recognition specificity and/or loses an old specificity. This will be done in a number of cases and all three types of recognition described above, will be studied. 2. The second is a population genetic approach. if a certain polymorphic sequence is responsible for a recognition epitope, it should always be associated with that epitope. The presence of a particular sequence will be assayed by specific oligonucleotide probes and compared with the DR recognition specificities of the same individual. I believe these studies will further our understnading of the structure/function relationships of class II molecules. If definite regions can be shown to control a particular recognition epitope, then these regions can be typed by molecular biological techniques. Such epitope specific typing would result in a greater finesse in HLA typing and would allow better prediction of successful donors for use in non-related transplants of bone marrow and other tissues and organs. It may also be applicable to the study of HLA-disease associations.